1. Field of the Invention
The present invention relates to a photothermographic material having an excellent coated surface and a reduced fogging, and an image forming method using the same.
2. Description of the Related Art
In recent years, decrease in the amount of processing liquid waste in the field of films for medical imaging has been keenly desired from the viewpoints of environmental protection and economy of space. For this reason, techniques regarding photothermographic materials for medical diagnosis and graphic arts, which can be exposed efficiently by laser image setters or laser imagers and can form clear black-toned images of high resolution and sharpness, are required. With the photothermographic materials described above, thermal development systems which do not require liquid processing chemicals, are simpler, and do not damage the environment can be supplied to customers.
While similar requirements also exist in the field of general image forming materials, images for medical imaging require a particularly high image quality excellent in sharpness and granularity since fine representation is required, and are characterized in that images of blue-black tones are preferred from the viewpoint of easy diagnosis. At present, various kinds of hard copy systems utilizing dyes or pigments such as ink jet printers and electrophotographic systems have been marketed as general image forming systems, but they are not satisfactory as output systems for medical images.
Thermal image forming systems utilizing organic silver salts are described in a number of documents. Photothermographic materials generally comprise an image forming layer in which a catalytically active amount of photo catalyst (for example, a silver halide), a reducing agent, a reducible silver salt (for example, an organic silver salt), and if necessary, a toning agent for controlling the color tone of silver, dispersed in a binder. A photothermographic material forms a black silver image by being heated to a high temperature (for example, 80° C. or higher) after imagewise exposure to cause an oxidation-reduction reaction between a silver halide or a reducible silver salt (functioning as an oxidizing agent) and a reducing agent. The oxidation-reduction reaction is accelerated by the catalytic action of a latent image on the silver halide generated by exposure. As a result, a black silver image is formed in the exposed region. Further, the Fuji Medical Dry Imager FM-DP L is has been marketed as a medical image forming system using a photothermographic material.
In the manufacture of thermographic image forming systems using organic silver salts, there is a method of manufacture by solvent coating, and a method of coating, with a coating liquid containing an aqueous dispersion of polymer fine particles of mainly binder, and drying. Because there is no need for a solvent recovery process in the later method the manufacturing facilities are simple, the environmental load is small, and it is advantageous for large scale production. However, because the coating liquid does not having a setting property, after application of the coating liquid the film can be distorted by the drying air, and there is the problem that drying blemishes can easily develop.
The use of hydrophilic binders such as gelatin has been proposed (see, for example, the publications of U.S. Pat. Nos. 6,630,291 and 6,713,241). But this is not usable in practice, since the thermographic activity is low, and when the activity is increased in order to try to obtain sufficient images there is the problem that there is a large amount of fogging.
With thermographic materials there is a need to include in advance within the film the chemical components necessary for image forming. Because of this, the chemical components influence the storage stability up to the time when the photothermographic material is used. Also, after forming of an image by thermal development, the chemical components remain within the film in an un-reacted state or as reaction products, and this has a great influence on the film transparency and image tone, and the image storage stability.
There is even more of a problem with these storage stabilities when the above hydrophilic binder is used then, and improvement is required.
The inventors have investigated photothermographic materials which can give a superior coating surface state, using as the binder in the image forming layer hydrophilic binders, such as gelatin, with setting characteristics. Conventionally, hydrophilic binders are use in wet development type silver halide photosensitive materials. However, when used as a binder for photothermographic materials a problem was discovered that never arose with wet development type silver halide photosensitive materials. The fundamental problem is, because the developing activity is extremely low, the image density is low, and the sensitivity is low. In order to increase the thermal developing properties, lowering the amount of hydrophilic binder in the image forming layer, that is, increasing the ratio of organic silver salt relative to the hydrophilic binder was tried. However, when the ratio of organic silver salt to hydrophilic binder was raised it generated greatly increased fogging, or worsening of the raw storage stability and image storage stability. Furthermore, the problem of contamination of unexpected portions of plates of the thermo developing portions and of heating elements, such as the drum, was encountered. The reason for the contamination is thought to be that volatile materials are generated at the time of thermal development from the photothermographic materials and these become adhered to the peripheral elements. As a result of extensive research into how to solve this new problem, the maintaining of the ratio of organic silver salt relative to the hydrophilic binder within a predetermined range was discovered, as a means of improving the developing activity, which resulted in this invention being arrived at. Also, by using the photothermographic materials used in the invention, an image forming method has been discovered providing rapid image formation, giving rise to the image forming method of the invention.